Fructose

Polychlorinated biphenyls (PCBs) are a class of organic compounds with 1 to 10 chlorine atoms
attached to biphenyl, which is a molecule composed of two benzene rings. PCBs were widely used
for many applications, especially as dielectric fluids in transformers, capacitors, and coolants. Due
to PCB's toxicity and classification as a persistent organic pollutant, PCB production was banned
by the United States Congress in 1979 and by the Stockholm Convention on Persistent Organic
Pollutants in 2001.
Aroclor 1260 has 12 carbon atoms and contains 60% chlorine by mass. An exception is Aroclor
1016, which also has 12 carbon atoms, but has 42% chlorine by mass. Different Aroclors were used
at different times and for different applications. In electrical equipment manufacturing in the USA,
Aroclor 1260 and Aroclor 1254 were the main mixtures used before 1950, Aroclor 1242 was the
main mixture used in the 1950s and 1960s until it was phased out in 1971 and replaced by Aroclor
1016.
Alternative names
Commercial PCB mixtures were marketed under the following names.:
Brazil
Former Czechoslovakia
France
Germany
Italy
Ascarel
Delor
Phenoclor
Pyraléne (both used by
Prodelec)
Clophen (used by Bayer)
History
PCBs, originally termed "chlorinated diphenyls," were commercially produced as complex mixtures
containing multiple isomers at different degrees of chlorination. In the United States, commercial
production of PCBs was taken over in 1929 by Monsanto Company from Swann Chemical Company.
Manufacturing levels increased in response to the electrical industry's need for a "safer" (than
flammable mineral oil) cooling and insulating fluid for industrial transformers and capacitors.
PCBs were also commonly used as stabilizing additives in the manufacture of flexible PVC coatings
for electrical wiring and electronic components to enhance the heat and fire resistance of the PVC.
The toxicity associated with PCBs and other chlorinated hydrocarbons, including polychlorinated
naphthalenes was recognized very early due to a variety of industrial incidents. A conference
about the hazards was organized at Harvard School of Public Health in 1937, and a number of
publications referring to the toxicity of various chlorinated hydrocarbons were published before
1940. Robert Brown reminded chemists in 1947 that Arochlors were "objectionably toxic. Thus the
maximum permissible concentration for an 8-hr. day is 1 mg/m3 of air. They also produce a serious
and disfiguring dermatitis". However, PCB manufacture and use continued with few restraints
until the 1970s.
PCBs are persistent organic pollutants and have entered the environment through both use
and disposal. The environmental transport of PCBs is complex and nearly global in scale. The
public, legal, and scientific concerns about PCBs arose from research indicating they were likely
carcinogens having the potential to adversely impact the environment and therefore undesirable as
commercial products. Despite active research spanning five decades, extensive regulatory actions,
and an effective ban on their production since the 1970s, PCBs still persist in the environment and
remain a focus of attention.
The only North American producer, Monsanto Company, marketed PCBs under the trade name
Aroclor from 1930 to 1977. These were sold under trade names followed by a 4 digit number. The
first two digits generally refer to the number of carbon atoms in the biphenyl skeleton (for PCBs this
is 12), the second two numbers indicate the percentage of chlorine by mass in the mixture. Thus,
Manufacture peaked in the 1960s, by which time the electrical industry had lobbied the U.S.
Congress to make them mandatory safety equipment. In 1966, they were determined by Swedish
chemist Dr. Soren Jensen to be an environmental contaminant, and it was Dr. Jensen, according to
a 1994 article in Sierra, who named them PCBs. Previously, they had simply been called "phenols"
or referred to by various trade names, such as Aroclor, Kennechlor, Pyrenol, Chlorinol and others.
Their commercial utility was based largely on their chemical stability, including low flammability,
and desirable physical properties, including electrical insulating properties. Their chemical and
physical stability has also been responsible for their continuing persistence in the environment,
and the lingering interest decades after regulations were imposed to control environmental
contamination.
In 1972, PCB production plants existed in Austria, the then Federal Republic of Germany, France,
Great Britain, Italy, Japan, Spain, USSR, and USA.
In 1973 the use of PCBs was banned in "open" or "dissipative" sources, such as:
• plasticisers in paints and cements
• casting agents
• fire retardant fabric treatments and heat stabilizing additives for PVC electrical insulation
• adhesives
• paints and water-proofing
• railway sleepers
However, they continued to be allowed in "totally enclosed uses" such as transformers and capacitors,
which, in certain failure modes or out-of-specification conditions, can leak, catch fire, or explode.
It was Ward B. Stone of the New York State Department of Environmental Conservation (NYSDEC)
who first published his findings in the early 1970s that PCBs were leaking from transformers and
had contaminated the soil at the bottom of utility poles.[citation needed] Concern over the toxicity
and persistence (chemical stability) of PCBs in the environment led the United States Congress to
ban their domestic production in 1979, although some use continues in closed systems such as
capacitors and transformers.
"Enclosed uses" of PCBs include:
• capacitors
• insulating fluids in transformers
• vacuum pump fluids
• hydraulic fluids
188 189